1. Field of the Invention
The present invention generally relates to a method and apparatus for protecting a solenoid and its associated driving circuit in an electronic device and more particularly to protecting a solenoid and the associated driving circuit in a point-of-sale terminal having a solenoid operated cash drawer driven by a circuit in a receipt printer.
2. Description of the Related Art
The use of software to control the operation of electronic business machines, such as point-of-sale (POS) terminals has become widespread. For example, software in these machines is typically used to generate a signal that energizes a solenoid to open a cash drawer.
A fault or malfunction in the software or intervening hardware may cause the signal to continue indefinitely, thereby causing the solenoid to remain energized. If the solenoid is energized for a period that exceeds its rated duty cycle, the driving circuit and the solenoid will likely be damaged. The user will then be required to spend a considerable amount of time and money to repair the cash drawer and/or receipt printer before the POS terminal is able to function properly again.
A partial block diagram of a conventional POS system 10 is shown in FIG. 1. Under normal operating conditions, application software residing in a computer 12 controls, among other functions, the opening of a cash drawer 14. Specifically, to open the cash drawer 14, a popular command issued by the computer 12 to a receipt printer 16 is the character CTRL-G, which is not a printable character.
The receipt printer 16 interprets CTRL-G as a command to output a pulse that opens the cash drawer 14. Typical parameters of the pulse include a duration T of 100–500 milliseconds and a peak voltage Vcc of 12–24 VDC, which is often applied to the base of a transistor 18 in the receipt printer 16 used to drive an energizing signal 20 between Nodes D1 and D2.
The cash drawer solenoid 22 typically has a relatively high Q or quality factor, which is the ratio of its reactance to it effective series resistance at a given frequency. Specifically, the inductance of the solenoid 22 is high but the resistance is low. Thus, for relatively short pulse durations, such as the preferred 100–500 millisecond energizing signal 20, the impedance of the solenoid 22 is high, which limits the current through the transistor 18 and solenoid 22 to an acceptable value. However, for pulses having a longer duration, the impedance of the solenoid becomes very low, which may appear as a short circuit to the driving transistor 18 in the receipt printer 16.
Burnout of the transistor 18 and/or the solenoid 22 occurs whenever the duration of the pulse exceeds a certain value. Burnout also occurs if the output of the transistor 18 is momentarily grounded, which may happen during system installation.
U.S. Pat. No. 5,111,394 to Hilles et al. (Hilles), which is incorporated herein by reference, describes an electronic business machine having a cash drawer and a timeout circuit. The timeout circuit couples the output of a microprocessor to a driver for a programmable period of time. The driver provides a signal that energizes a solenoid to open the cash drawer. In an alternative embodiment, a multivibrator is substituted for the timeout circuit.
Hilles overcomes the problem of damaged transistors and solenoids common in conventional POS systems by limiting the period of time that the solenoid is energized. However, in the first embodiment of Hilles, the timeout circuit requires over thirty digital devices that add to the cost and require a significant amount of board space, as well as a source of power. Although the second embodiment of Hilles implements the timeout function in a single integrated device, that is, a multivibrator, such an active device still represents an unreliable and significant additional cost while requiring its own source of power. These characteristics substantially limit the location of and applications for the solutions proposed in Hilles.
It is an objective of the present invention to provide a much simpler and more reliable method and apparatus for protecting a solenoid and/or receipt printer in an electronic device, such as a point-of-sale terminal, which uses inexpensive passive components that do not require a source of power.
It is another objective of the present invention to provide a method and apparatus for protecting a solenoid and/or receipt printer in an electronic device, such as a point-of-sale terminal, that requires only a small amount of space and consumes negligible power.
It is yet another objective of the present invention to provide a method and apparatus for protecting a solenoid and/or receipt printer in an electronic device, such as a point-of-sale terminal, which can be used to retrofit existing electronic devices, as well as being designed into new equipment.
It is still another objective of the present invention to provide a method and apparatus for protecting a solenoid and/or receipt printer in an electronic device, such as a point-of-sale terminal, that employs a simple, straightforward design that is reliable, easy to test, and relatively maintenance free.